对以上3点,如果你深有理会,如下内容可以不看,不要浪费时间。如果没有很深的概念,可以参考一下。如有问题,希望在评论区或其他方式给我指点,谢谢。
非继承类的析构函数执行会有如下操作:
1、执行析构函数中的代码
2、如果所在类有成员变量(类对象),再执行类成员变量的析构函数
普通析构函数:
以如下代码为例
class Student
{
public:
Student()
{
name = new char[32];
cout << "Student constructor new name char[32]" << endl;
}
~Student()
{
delete name;
cout << "Student destructor delete Student::name" << endl;
}
private:
char* name;
};
class Normal
{
public:
Student stu;
Normal(){cout << "Normal constructor" << endl;}
~Normal(){cout << "Normal destructor" << endl;}
};
int main()
{
Normal* pn = new Normal();
delete pn;
return 0;
}
执行结果:
(表示很讨厌上面的水印)
析构顺序:
1、执行析构函数中的代码
2、如果所在类有成员变量(类对象),再执行类成员变量的析构函数
以上内容很好理解,因为只是单个类对象的析构,当然也关系到类的组合。
接下来我想讨论的是存在继承关系时,析构函数在做些什么?
#include
using namespace std;
#include
class Student
{
public:
Student()
{
name = new char[32];
cout << "Student constructor new name char[32]" << endl;
}
~Student()
{
delete []name;
cout << "Student destructor delete Student::name" << endl;
}
private:
char* name;
};
class Base
{
public:
Base()
{
cout << "Base()" << endl;
}
~Base()
{
cout << "~Base()" << endl;
}
};
class Derived:public Base
{
public:
Derived()
{
cout << "Derived()" << endl;
}
~Derived()
{
cout << "~Derived()" << endl;
}
Student stu;
};
int main()
{
Derived *pd = new Derived();
delete pd;
return 0;
}
执行结果:
派生类对象的析构过程:
1,调用派生类对象的析构函数,
2,调用派生类中成员对象的析构函数
3,调用基类的析构函数
-------------------------------------我是丑陋的分割线-------------------------------------------
在分割线的上方,一切正常,没有什么特别的,异样的地方。
接下来要讨论的是多态情况下的析构函数。
#include
using namespace std;
#include
class Student
{
public:
Student()
{
name = new char[32];
cout << "Student constructor new name char[32]" << endl;
}
~Student()
{
delete []name;
cout << "Student destructor delete Student::name" << endl;
}
private:
char* name;
};
class Base
{
public:
Base()
{
cout << "Base()" << endl;
}
~Base()
{
cout << "~Base()" << endl;
}
};
class Derived:public Base
{
public:
Derived()
{
cout << "Derived()" << endl;
}
~Derived()
{
cout << "~Derived()" << endl;
}
Student stu;
};
int main()
{
Base* pb = new Derived(); // 注意这一行,基类指针指向了派生类
delete pb; // delete 基类指针
return 0;
}
执行结果:
只调用基类的析构函数,派生类的析构函数没有被调用,更要注意的是,因此派生类的成员对象也没有被析构,缺少了“Student destructor delete Student::name”
所以会有内存泄漏。
如下代码也会出现内存泄漏:
class Base
{
};
class Derived:public Base
{
public:
string str; // 这种内存泄漏就不容易发现了!!!
};
int main()
{
Base* pb = new Derived(); // 派生类的string对象会发生内存泄漏!
delete pb;
return 0;
}
如上类继承时发生的内存泄漏,如何解决?
只需要将基类的析构函数声明为虚析构函数,就是前面加上virtual.
#include
using namespace std;
#include
class Student
{
public:
Student()
{
name = new char[32];
cout << "Student constructor new name char[32]" << endl;
}
~Student()
{
delete []name;
cout << "Student destructor delete Student::name" << endl;
}
private:
char* name;
};
class Base
{
public:
Base()
{
cout << "Base()" << endl;
}
virtual ~Base()
{
cout << "~Base()" << endl;
}
};
class Derived:public Base
{
public:
Derived()
{
cout << "Derived()" << endl;
}
~Derived()
{
cout << "~Derived()" << endl;
}
Student stu;
};
int main()
{
Base* pb = new Derived();
delete pb;
return 0;
}
运行结果:
发现一切OK了。
总结:
1.所有基类的析构函数,都应该声明为虚析构函数!这也是c++标准所指定的。
2.如果设计一个类,可能会被后来的其他类所继承